Plant for the production of energy based upon the organic rankine cycle

ABSTRACT

A plant for the production of energy that is based upon the organic Rankine cycle (ORC). The plant comprises a first ORC system, comprising a first organic operating fluid circulating, in sequence, between a first evaporator in conditions of heat exchange with a heat source, a first expansion stage in a turbine operatively connected to a generator, a first evaporator/condenser, and a first pump for recirculating said first organic operating fluid to said first evaporator. Said turbine is a partializable turbine and comprises means for partializing the incoming flowrate of said organic operating fluids, said means being designed to partialize said incoming flowrate to keep the r.p.m. of said turbine constant.

The present invention relates to a plant for the production of energybased upon the organic Rankine cycle (ORC), in particular a plant forthe production of energy comprising a plurality of cascaded ORC systemsthat use particular turbines.

As is known, ORC plants are systems generally used for simultaneousproduction of electrical and thermal energy, the latter being madeavailable in the form of water at the temperature of 60-90° C. OrganicRankine cycles are similar to the cycles used by traditional steamturbines, except for the operating fluid, which, normally, is an organicfluid with high molecular mass.

A typical ORC plant is substantially made up of a pump, a turbine, andsome heat exchangers. The organic operating fluid is vaporized by usinga heat source in the evaporator. The steam of the organic fluid expandsin the turbine and is then condensed generally using a flow of water ina heat exchanger. The condensed liquid is finally sent via a pump intothe evaporator thus closing the cycle. In order to increase the yield ofthe plant it is possible to envisage the use of a regenerator. In thiscase, the fluid leaving the turbine traverses a regenerator before beingcondensed and, once condensed, is pumped into the regenerator, where itis pre-heated by the fluid leaving the turbine, before being sent to theevaporator.

Generally, these plants are used for the production of energy with wastefluids coming from a wide range of industrial and energy-generatingprocesses (cogeneration engines and turbines, furnaces of all types,chimneys of petrochemical plants, sources of geothermal nature, etc.),characterized by a temperature jump that is potentially high but byflows that are on average limited or in any case variable in time or,otherwise, by high flows associated, however, to a low temperaturelevel.

The energy vector used for vaporization of the organic fluid is ingeneral diathermic oil (mineral oil, or synthetic oil for temperaturesabove 300° C.) or water, whereas for condensation water is used. The useof diathermic oil moreover prevents the need to use high-pressureboilers.

The operating fluid is generally constituted by an organic compound orby a mixture of organic compounds, characterized by a high molecularweight. The choice of the organic fluid to be used for optimizing theyield of the thermodynamic cycle is made according to the temperature ofthe heat source available. In addition, also the turbine must normallybe designed according to the characteristics of the organic fluid and tothe operating conditions.

Consequently, it is evident that the application of ORC systems issubject to limits and critical aspects deriving from the thermal poweravailable as primary source, from the temperature level/levels (qualityof the source), and from the stability and/or relative variability intime of the thermal load.

It is moreover evident that current ORC systems are far from flexibleand far from adaptable to different operating conditions given that, aspreviously mentioned, both the choice of the organic operating fluid andthe characteristics of the turbine are markedly dependent upon theoperating conditions.

The purpose of the present invention is consequently to provide a plantfor the production of electrical energy based upon the organic Rankinecycle (ORC) that will be able to eliminate or reduce the aforementioneddrawbacks.

In particular, a purpose of the present invention is to provide a plantof an ORC type for the production of electrical energy that will enablemaximization of the yield in terms of electrical energy produced.

Another purpose of the present invention is to provide a plant of an ORCtype for the production of electrical energy that will be readilyadaptable to different operating conditions. Yet another purpose of thepresent invention is to provide a plant of an ORC type for theproduction of electrical energy that will enable optimal characteristicsof yield to be maintained even during a drop inefficiency of the primaryflow.

A further purpose of the present invention is to provide a plant of anORC type for the production of electrical energy that will present asmall number of parts, and will be easy to produce at competitive costs.

In accordance with the present invention, the above-mentioned purposesare achieved by a plant for the production of energy based upon theorganic Rankine cycle (ORC), which is characterized in that it comprisesa first ORC system comprising a first organic operating fluidcirculating, in sequence, between a first evaporator in conditions ofheat exchange with a heat source, a first expansion stage in a turbineoperatively connected to a generator, a first evaporator/condenser, anda first pump for recirculating said first organic operating fluid tosaid first evaporator. A further peculiar characteristic of the plantaccording to the invention is represented by the fact that said turbineis a partializable turbine comprising means for partializing theincoming flowrate of said organic operating fluids, said means beingdesigned to partialize said incoming flowrate to maintain the r.p.m. ofsaid turbine constant.

Preferably, the plant for the production of energy according to theinvention comprises a second ORC system, which comprises a secondorganic operating fluid circulating, in sequence, between said firstevaporator/condenser, a second expansion stage in a turbine operativelyconnected to a generator, a second evaporator/condenser, and a secondpump for recirculating said second organic operating fluid to said firstevaporator/condenser, said turbine comprising, for each of said stages,means for partializing the incoming flowrate of said organic fluids.

The plant according to the invention enables a considerable series ofadvantages to be achieved as compared to the ORC systems of a knowntype.

In practice, in the case of the multistage organic Rankine cycle, thesystem is characterized by a series of successive enthalpic jumpsperformed not by a single fluid within various rotor-stator assembliesof a turbine (e.g., the water vapour that expands at various levels ofpressure in the stages of a turbine), but by a number of fluids, eachoperating on a number of pressure and temperature levels, within theirown turbine, which is coupled, axially aligned or in parallel, to theturbine for the other fluids that together with it constitute thesystem. The plant hence comprises a so-called “primary” fluid, whichinterfaces with the heat source, and an appropriate number of secondaryfluids, ordered in such a way that the condensation of the previous onecauses evaporation of the next one, in order to recover the maximumpossible amount of energy available to the source, yielding the minimumfraction thereof into the environment.

The limit of quality of the source (e.g., low enthalpic level) obviouslydictates the limits of application of the technology. The availabilityand the temperature of the cooling fluid define, instead, thelower-limit stage, enabling, with the use of organic fluids and binarymixtures for low-temperature applications, extension of the dischargefrom the last turbine to levels theoretically lower than thermal zero.

By way of example, for the most frequent cases of high temperature andlow flowrate of the power source it is preferable for the operatingfluid of the primary circuit to have a rather high molecular weight soas to exploit to the full the high temperatures that can present atdischarge (typically, 500-900° C.). The flowrate of said fluid is,however, in this case limited by the thermal power effectivelyavailable, and this circumstance is a first limiting factor on the powerproduced by the primary fluid.

A second factor, which is no less important, is the molecular weightitself of the fluid. A high molecular weight, which advantageouslyenables the high temperatures of the heat source to be pursued, proves apenalizing factor in terms of enthalpic jump in the turbine andcondensation temperature. For example, if this fluid at the evaporatoris able, with not excessively high pressures (20-40 bar), to vaporize atquite high temperatures (250-350° C.), at the condenser, albeit withpressures far higher than 1 bar, still comes out at rather hightemperatures, in the region of 160-250° C. Hence, with the firstoperating fluid, with a low flowrate, and a limited enthalpic jump inthe turbine, a low power is normally obtained, in the region of 15% ofthe one available at discharge.

However, the still significantly high temperatures of the primary fluidundergoing condensation, and the phase thereof selected for heatexchange with very high coefficients of transmission, enable use of asecondary operating fluid that will recover entirely or partially theheat of condensation of the first fluid, and will perform an altogetherindependent Rankine cycle, producing a further, significant amount ofelectric power. In fact, assuming the value of 15% referred topreviously as yield of the primary cycle means that of 100 kW at inputfor the first fluid there remain 85 kW available for the second organicoperating fluid. Choosing the latter with characteristics such as to beoptimal for working at lower operating temperatures and with a range ofpressures similar to those of the first fluid, said fluid will operateon lower isotherms, guaranteeing in any case a similar yield. Assumingthen again a 15% recovery means that, out of 85 kW available, a further12.75 kW are obtained, for a total of 27.75 kW, which is the theoreticalyield of a plant that works with just two “cascaded” fluids.

Hence, extrapolating the concept, there may be readily envisaged thepossibility of using a third fluid, or in general “n” fluids, which, byworking in cascaded fashion, i.e., as has been said, with theevaporation on the condensation of the previous one, optimize theentire, process limiting to a minimum the heat yielded by the lastcondensation, which is the one that is then discharged into theenvironment, and thus maximizing the total yield.

In the plant for the production of energy according to the invention,preferably said means for partializing the incoming flowrate of saidorganic operating fluids comprise, for each of said stages, a hydraulicdevice driven by the corresponding organic operating fluid.

In addition, advantageously, said partializable turbine is a multistageturbine, with single shaft or separate shafts, axially aligned or withparallel axes.

A preferred embodiment of the plant for the production of energyaccording to the present invention envisages that said heat source willcomprise heat-accumulation means.

For example, said heat-accumulation means can comprise a refractory-massaccumulation system or a molten-salt closed-circuit battery.

A further preferred embodiment of the plant for the production of energyaccording to the present invention envisages that said heat sourcecomprises means for integration of the available energy.

For example, said means for integration of the available energy comprisea thermodynamic solar-energy system.

A particular embodiment of the plant for the production of energyaccording to the present invention envisages the presence of a devicefor mechanical coupling between said turbine and said generator.

Said device for mechanical coupling between said turbine and saidgenerator can, for example, comprise a reducer, a flywheel governor, anda brake set between the shaft of said turbine and the shaft of saidgenerator.

An alternative embodiment of the plant for the production of energyaccording to the present invention envisages, instead, that said turbineis directly coupled to said generator, said plant further comprisingelectronic means for conversion of the output voltage of said generator.

Further characteristics and advantages of the present invention willemerge from the description of preferred, though not exclusive,embodiments of a plant for the production of energy based upon theorganic Rankine cycle (ORC), illustrated by way of non-limiting examplein the annexed drawings, in which:

FIG. 1 shows a diagram of a general embodiment of a plant according tothe present invention;

FIG. 2 shows a diagram of a first particular embodiment of a plantaccording to the present invention;

FIG. 3 shows a diagram of a second particular embodiment of a plantaccording to the present invention;

FIG. 4 is a schematic representation of a first embodiment of themechanical coupling between a turbine and a generator in a plantaccording to the present invention; and

FIG. 5 is a schematic representation of an embodiment of the coupling tothe electrical mains of a plant according to the present invention.

With reference to the attached figures, a plant for the production ofenergy based upon the organic Rankine cycle (ORC) according to thepresent invention, designated as a whole by the reference number 1, inits more general embodiment illustrated in FIG. 1, comprises at leastone first ORC system 10.

Said first ORC system 10 in turn comprises a first organic operatingfluid that circulates, in sequence, between a first evaporator 11 inconditions of heat exchange with a heat source 2, a first expansionstage 12 in a turbine operatively connected to a generator 4, a firstevaporator/condenser 13 and a first pump 14 for recirculating said firstorganic operating fluid to said first evaporator 11.

Preferably, as illustrated in FIG. 2, the plant 1 for the production ofenergy according to the present invention, comprises a second ORC system20, which in turn comprises a second organic operating fluidcirculating, in sequence, between said first evaporator/condenser 13, asecond expansion stage 22 in a turbine operatively connected to agenerator 5, a second evaporator/condenser 23, and a second pump 24 forrecirculating said second organic operating fluid to said firstevaporator/condenser 13, said turbine comprising, for each of saidstages 12 and 22, means for partializing the incoming flowrate of saidorganic fluids

Basically, as has already been mentioned, the plant comprises at leasttwo organic operating fluids, which, working in cascaded fashion(namely, with the evaporation of the second fluid on the condensation ofthe first fluid), optimize the entire process, limiting to a minimum theheat yielded by the last condensation.

In other words, using this scheme, the closed-circuit organic Rankinecycle uses the primary source of energy for converting the first fluidinto steam, the expansion in the turbine converts this heat accumulatedby the steam into kinetic energy, which in turn will become electricalenergy. In the cascade the condenser becomes the primary source for thesecond fluid, and so forth for a possible third stage and subsequentstages.

By applying the principle illustrated in the attached FIG. 2, it is infact possible to extend said concepts to a plant that comprises a thirdORC system, which includes a third organic operating fluid thatcirculates, in sequence, between said second evaporator/condenser, athird expansion stage in a turbine operatively connected to acorresponding generator, a third evaporator/condenser, and a third pumpfor recirculating said first organic operating fluid to said secondevaporator/condenser. In general, this principle can then be extended toa plant that comprises “n” further ORC systems.

One of the peculiar characteristics of the plant 1 according to thepresent invention is represented by the fact that said turbine is apartializable turbine and comprises means for partializing the incomingflowrate of said organic operating fluids. In particular, saidpartialization means are designed to partialize said inlet flowrate tomaintain the r.p.m. of said turbine constant.

It has in fact been seen that, using a turbine of the type describedabove, it is possible to maintain the optimal characteristics of yieldeven during a drop of efficiency of the primary flow. Thanks to theautomatic partialization of the flowrate, the r.p.m. of the turbine is,in fact, kept constant, which is an extremely important aspect forachieving maximum yield.

In practice, partialization of the turbine through the control ofincoming flowrate in order to keep the r.p.m. of the shaft constant,enables the level of electrical yield to be kept constant irrespectiveof the load, thus contributing to the maximum exploitation of the sourceof thermal power in every condition that arises upstream.

The application of a process of a cascaded type finds justification inthe very nature of the heat exchange and of the characteristics of thepower fluid. In fact, it is precisely the first exchanger of the system,the evaporator of the primary fluid with highest enthalpy, the one thatis most critical given that it is interfaced with the waste fluids,which each time present low coefficients of heat exchange, highcorrosiveness, and non-constancy of the flowrate.

Using a cascaded system, it is possible to concentrate the process ofheat recovery in this first exchanger, which constitutes the centralelement of the entire cycle both in terms of efficiency and in terms ofcosts.

The subsequent condensers-evaporators are favoured by the cascadedfunctional logic and by the nature of the fluids adopted, both in theefficiency of interchange and in the nature of the design andmanufacturing materials required, thus involving small surfaces andhence low costs.

Preferably, said means for partializing the incoming flowrate of saidorganic operating fluids comprise a hydraulic device driven by thecorresponding organic operating fluid. In this way, the flowrate atinlet to said organic operating fluids is adjusted exploiting thevariations of pressure of the organic fluids themselves.

In addition, advantageously, said partializable turbine can be amultistage turbine, with single shaft or with separate shafts, axiallyaligned or with parallel axes.

A particular embodiment of the plant 1 for the production of energyaccording to the present invention, illustrated in FIG. 3, ischaracterized in that said heat source 2 comprises heat-accumulationmeans 6.

In the case where the plant of the present invention is applied toindustrial processes with production of intermittent waste in the shortperiod, there may arise in fact the need to stabilize the thermal flowat levels compatible with the stepwise regulation of the plant.

There may consequently arise the following situations:

-   -   expedience of limiting the load with re-use of part thereof for        optimization of the consumption correlated to the basic        production processes;    -   expedience, if not strict need for regulation of the ORC unit        downstream, of proceeding to a partial accumulation of the heat        available to stabilize the thermal flow at homogeneous levels.

By using the heat-accumulation means 6, which comprise, for example, arefractory-mass accumulation system or a molten-salt closed-circuitbattery, it is possible to maintain an average thermal level guaranteedof the heat source 2, managing the situations described above in anoptimal way.

For low thermal powers a refractory-mass accumulation system with lowimpact from the economic and maintenance standpoints is preferable. Inthis case, the hot process fluid traverses the refractory material untilits own thermal load is reduced to the value at input to thehigh-temperature evaporator. When the flowrate of the waste fluid isreduced and/or is about to cease, a closed-circuit circulation of hotair (or recirculation of the gas itself) is activated between theaccumulation chamber and the evaporator. For applications of this type,the optimal accumulation temperature range is between 200° C. and 400°C.

For high powers, the accumulation system may conveniently be based upona closed-circuit molten-salt battery, also according to the economicaspects of the investment required for the accumulation tank in thelight of the effective thermal capacity that can be recovered and henceof the corresponding electrical production.

The aggressiveness of the molten salts requires a limitation of thetemperature of the mixtures (nitrates and nitrides of sodium, potassiumand calcium) to values of 400-450° C. to enable use of low-costmaterials for heat exchangers and tanks. Furthermore, to preventcombination of the effects of aggressiveness of the salts with thesource fluid it is expedient to use an intermediate vector fluid of adiathermic type. Finally, the choice of technologies of exchangers thatensure low loss of head for viscous fluids (‘EM-Baffle®’ technology, andthe like) completes the requirements of the system.

A further particular embodiment of the plant 1 for the production ofenergy according to the present invention, illustrated once again inFIG. 3, envisages that said heat source 2 comprises means 7 forintegration of the available energy.

Both using the refractory-mass system and the molten-salt system, aslikewise other accumulation systems, it is in fact possible to integratethe power available so as to ensure the enthalpic jump within theorganic Rankine cycle. Said integration can be obtained through re-useof part of the electrical power produced for stabilizing the cycleand/or achieving maximum thermodynamic yield thereof. It is likewisepossible to increase the absolute power thereof through the use ofparallel sources according to the time of day and the demand.Specifically, technologies applied to the thermodynamic solar sectorcan, for example, be used to increase the available flowrate adequatelyduring periods of peak demand.

Coupling of the energy-integration means to the power-accumulation meansensures maximum flexibility of the entire cycle, duly exploiting thestepwise operation of the turbine, as illustrated previously in the caseof cascaded multistage organic Rankine cycles. Both the accumulationmeans 6 and the integration means 7 can in fact be conveniently appliedto plants comprising a plurality of organic Rankine cycles of the typeillustrated in FIG. 2.

The plant 1 for the production of energy according to the presentinvention can moreover comprise means for electrical transduction,namely, for the transformation of the kinetic energy developed by theturbine into electrical energy.

For example, with reference to the attached FIG. 4, the plant 1 for theproduction of energy according to the present invention canadvantageously comprise a mechanical-coupling device 8 between saidturbine and said generator 4, 5.

Said mechanical-coupling device 8 can, for example, comprise a reducer81 set between the shaft 84 of said turbine and the shaft 85 of saidgenerator 4, 5. Said reducer 81 can, for example, be a reducer of anepicyclic type in such a way as to guarantee reduction of r.p.m. fromthe turbine to the generator (alternator), thus maintaining the rightfrequency for output the energy produced to the mains.

Furthermore, the reducer 81 conveniently comprises all the elements (forexample, a flywheel governor 82 and a brake 83) that will enablemaintenance of the correct working points of the entire system. Theconnection with the mains can be made via a transformer 86 and/or othermeans for synchronization with said mains.

Alternatively, as illustrated in FIG. 5, said turbine is directlycoupled to said generator 4, 5. For example, the turbine can be directlycoupled to a (synchronous or asynchronous) motor, eliminating the entiremechanical block described previously. In this case, said device can beused in the four quadrants (Vx1>0; Vx1<0) both as motor for entering thetransients of the turbine and exiting therefrom and as electricgenerator.

In this case, electronic means 9 are present, for example an AC/DCconverter for continuous conversion of the high-frequency voltage(frequencies much higher than 50 Hz) given by the direct coupling withthe turbine, as well as a DC/AC converter to obtain the right outputvoltage with the appropriate synchronism for output into the mains.Transformer means 91 may likewise be conveniently present.

It is clear from the foregoing description that the plant for theproduction of energy according to the present invention fully achievesthe pre-set task and purposes.

The partialization of the turbine responds in fact to the scenarios ofthermal load that can vary according to pre-defined steps, wherealternative options of use of the power are available or, more directly,for adapting to reduction of the demand, whatever the origin of saidreduction. Said application consequently constitutes an effectiveresponse, for example in the cases of cogeneration coupled topartializable generators, the cycle of which finds itself pursuing theload in steps (down to a minimum of less than 50% of the nominal size),and/or enabling an increase in the global production of electricity todestine the entire thermal load available to said production (e.g.,absence of use or reduced use of hot water/steam in non-winter periodsand non-primary interest in trigeneration).

Coupling with a secondary heat source (e.g., solar concentration source)moreover enables exploitation of the turbine at its maximum level ofpower, reducing the primary consumption or simply increasing the globalproduction of electricity in the periods of increasing demand. Finally,the accumulation of power enables balancing of the production in timeand, once again, maximization of the production within the limit of thepower effectively available, during periods of increasing demand.

From the foregoing it may hence be stated that the plant for theproduction of energy according to the present invention, in particularwhen it combines cascaded organic Rankine cycles, downstream of anaccumulation-integration plant, with partializable turbines, presents asideal from the technical and economic standpoints for maximization ofthe yield of production of electrical energy from waste and/or sourcesof heat of medium-to-low absolute power of any nature.

On the basis of the foregoing description, other characteristics,modifications, or improvements are possible and evident to the averageperson skilled in the sector. Said characteristics, modifications, andimprovements are hence to be considered as forming part of the presentinvention. In practice, the materials, used as well as the contingentdimensions and shapes, may be any according to the requirements and thestate of the art.

1. A plant for the production of energy based upon the organic Rankinecycle (ORC), wherein it comprises a first ORC system comprising a firstorganic operating fluid circulating, in sequence, between a firstevaporator in conditions of heat exchange with a heat source, a firstexpansion stage in a turbine operatively connected to a generator, afirst evaporator/condenser, and a first pump for recirculating saidfirst organic operating fluid to said first evaporator; said turbinebeing a partializable turbine and comprising means for partializing theincoming flowrate of said organic operating fluids, said means beingdesigned to partialize said inlet flowrate to maintain the r.p.m. ofsaid turbine constant.
 2. The plant for the production of energyaccording to claim 1, wherein said means for partializing the incomingflowrate of said organic operating fluids comprise a hydraulic devicedriven by the corresponding organic operating fluid.
 3. The plant forthe production of energy according to claim 1, wherein it comprises asecond ORC system, comprising a second organic operating fluidcirculating, in sequence, between said first evaporator/condenser, asecond expansion stage in a turbine operatively connected to agenerator, a second evaporator/condenser, and a second pump forrecirculating said second organic operating fluid to said firstevaporator/condenser, said turbine comprising, for each of said stages,means for partializing the incoming flowrate of said organic fluids. 4.The plant for the production of energy according to claim 1,wherein saidpartializable turbine is a multistage turbine, with single shaft or withseparate shafts, axially aligned or with parallel axes.
 5. The plant forthe production of energy according to claim 1, wherein said heat sourcecomprises heat-accumulation means.
 6. The plant for the production ofenergy according to claim 5, wherein said heat-accumulation meanscomprise a refractory-mass accumulation system or a closed-circuitmolten-salt battery.
 7. The plant for the production of energy accordingto claim 1, wherein said heat source comprises means for integration ofthe available energy.
 8. The plant for the production of energyaccording to claim 7, said means for integration of the available energycomprise a thermodynamic solar-energy system.
 9. The plant for theproduction of energy according to claim 1, wherein it comprises a devicefor mechanical coupling between said turbine and said generator.
 10. Theplant for the production of energy according to claim 9, wherein saidmechanical-coupling device between said turbine and said generatorcomprises a reducer, a flywheel governor, and a brake set between theshaft of said turbine and the shaft of said generator.
 11. The plant forthe production of energy according to claim 1, wherein said turbine isdirectly coupled to said generator, said plant comprising electronicmeans for conversion of the output voltage of said generator.
 12. Theplant for the production of energy according to claim 2, wherein itcomprises a second ORC system, comprising a second organic operatingfluid circulating, in sequence, between said first evaporator/condenser,a second expansion stage in a turbine operatively connected to agenerator, a second evaporator/condenser, and a second pump forrecirculating said second organic operating fluid to said firstevaporator/condenser, said turbine comprising, for each of said stages,means for partializing the incoming flowrate of said organic fluids. 13.The plant for the production of energy according to claim 2 wherein saidpartializable turbine is a multistage turbine, with single shaft or withseparate shafts, axially aligned or with parallel axes.
 14. The plantfor the production of energy according to claim 3 wherein saidpartializable turbine is a multistage turbine, with single shaft or withseparate shafts, axially aligned or with parallel axes.
 15. The plantfor the production of energy according to claim 2, wherein said heatsource comprises heat-accumulation means.
 16. The plant for theproduction of energy according to claim 3, wherein said heat sourcecomprises heat-accumulation means.
 17. The plant for the production ofenergy according to claim 4, wherein said heat source comprisesheat-accumulation means.
 18. The plant for the production of energyaccording to claim 2, wherein said heat source comprises means forintegration of the available energy.
 19. The plant for the production ofenergy according to claim 3, wherein said heat source comprises meansfor integration of the available energy.
 20. The plant for theproduction of energy according to claim 4, wherein said heat sourcecomprises means for integration of the available energy.